1. Field of the Invention
The present invention generally relates to the field of communications network and, particularly, to methods and systems for controlling the access to a shared communication medium. Even more particularly, the invention relates to communication media access control methods and systems for ad-hoc communication networks.
2. Description of the Related Art
Ad-hoc communication networks are a type of mobile, wireless communication networks that, differently from cellular networks, do not need (at least in principle) any fixed and wired communication network infrastructure (e.g. terrestrial base stations) for routing the communication traffic. For this reason, while cellular networks can be defined as infrastructured mobile wireless communication networks, ad-hoc networks can conversely be referred to as infrastructureless mobile wireless communication networks.
Such a goal is achieved thanks to the fact that in an ad-hoc network the network nodes are formed by mobile hosts, which are capable of movement and act as network routers, cooperating to the routing of the communication traffic. In particular, the ad-hoc network nodes communicate with each other either directly, if they are within the respective communication ranges, or, if they are not within the direct communication range, indirectly, in a multi-hop fashion, relying on one or more intermediate nodes for the outing of the communication traffic.
Consequently, in an ad-hoc network each node not only is a communication network user (or station), but it may also act as a communication traffic router and cooperates with other nodes for maintaining the network connectivity. Ad-hoc networks are thus dynamically formed by the very network users (or stations).
Ad-hoc networks are particularly useful in applications wherein a network infrastructure does not exist and setting it up is not feasible, for example for time constraints, or not convenient, for example in view of the costs to be sustained. Situations in which it may be useful to rely on an ad-hoc network for enabling communications include battlefields, areas hit by natural disasters, rescue missions, conferences, meetings, and, last but not least, inter-vehicle communications.
Despite the advantages that ad-hoc networks exhibit compared to infrastructured networks, there still are problems to be faced and solved before this type of network starts to be widely deployed. Needless to say, most of these problems are inherent to the very nature of ad-hoc networks, i.e. the mobility of the network nodes.
In the art, most of the problems of ad-hoc networks have already been recognized.
For example, in K. H. Wang et al., “Group Mobility and Partition Prediction in Wireless Ad-Hoc Networks”, Proceedings of IEEE International Conference on Communications (ICC), 2002, the problem is dealt with of service disruptions due to network partitioning, that is, the separation of an ad-hoc network into completely disconnected portions, as a consequence of the mobility of the network nodes. The authors of that paper propose a velocity-based group model that characterizes the movements of the mobility groups, in order to predict network partitioning; the nodes' velocities are obtained via GPS (Global Positioning System).
As another example, in L. Briesemeister et al., “Overcoming Fragmentation in Mobile Ad Hoc Networks”, Journal of Communications and Networks, 2(3):182-187, September 2000, an approach is presented for a location-based multicast of emergency messages among hosts in ad-hoc networks, like vehicles in road traffic, according to which a multicast group is defined implicitly by time, location (determined exploiting GPS vehicle navigation systems), speed and driving direction.
In J. P. Singh et al., “Wireless LAN Performance Under Varied Stress (Conditions in Vehicular Traffic Scenarios”, IEEE Vehicular Technology Conference, Fall 2002, vol. 2, pp. 743-747, Vancouver, the performance of a 802.11b-based Wireless LAN (WLAN) for inter-vehicle communications in different vehicular mobility, peer-distance and driving environment conditions have been assessed, and it has been observed that the performance degrade with increasingly hostile communication scenarios; in particular, in that paper it is pointed out that the urban driving conditions are the most severe environment, while the sub-urban environment is the most favorable, and the freeway environment lies between the above two.
The Applicant has observed that in ad-hoc networks the problems of collisions between different users, that arise in situations where a shared communication medium is exploited, and the communication medium (typically a radio communication medium) is partitioned into a plurality of distinct portions (hereinafter referred to as transmission resources or communication channels) are exacerbated.
Several different ways exist of partitioning a shared communication medium into a plurality of distinct communication channels. One known way is referred to as Time Division Multiple Access (TDMA) and allows several users sharing a same frequency by dividing it into different time slots. TDMA is for example used in the General System for Mobile communications (GSM) digital cellular standard, among others. Another known way of partitioning a shared communication medium is referred to as Code Division Multiple Access (CDMA), and uses spread spectrum techniques by multiple transmitters to send messages to a same receiver on a same frequency channel at a same time without harmful interference. Still another known technique, frequently used in combination with any of the previous two techniques, is called Frequency Division Multiple Access (FDMA): in this case, each transmitter is assigned a distinct frequency channel, so that different receivers can discriminate among them by tuning onto the desired channel.
According to the Open Systems Interconnect (OSI) “layer stack” model set forth by the International Organization for Standardization (ISO), access by a network node to the shared communication medium is governed by the Media Access Control (MAC), which is the lower sub-layer of the data link layer, the interface between a logical link control and a physical layer. In addition to controlling the access to the shared communication medium, the MAC sub-layer is also concerned with breaking data up into data frames, transmitting the frames sequentially, processing the acknowledgment frames sent back by the receiver, handling address recognition.
The specific implementation of the MAC sub-layer depends, among other things, on the technique used for partitioning the shared communication medium into channels: thus, TDMA, CDMA and FDMA MAC sub-layers exist.
Generally, in a communication network a collision between two (or more) users is said to take place when the two users, both needing to transmit, access the shared communication medium exploiting a same transmission resource, i.e. a same communication channel; for example, in the case of a TDMA partitioning, a collision between two users takes place if the two users exploit the same time slot. In this situation, the physical layer of a listening user detects the presence of transmission over that communication channel (e.g., in that time slot), but the MAC layer cannot receive correctly a data block.
In an ad-hoc network, the problem of collisions is particularly felt, due to the very fact that the network architecture evolves dynamically; the relative movement of the various network nodes causes a more or less continuous change of scenario, thereby nodes that were previously within the respective communication ranges exit therefrom, and new nodes enter thereinto.
The Applicant has observed that the problems discussed above tends to be exacerbated as the relative velocity of the nodes increases, and that a particularly critical situation is encountered in ad-hoc networks for inter-vehicle communications in highway/motorway contexts: in these cases, two (more generally, a discrete, limited number of) flows of vehicles exist, corresponding to the two opposite ride directions; while the relative velocity of vehicles riding in the same direction is, on average, relatively low, the relative velocity between vehicles that ride in the two opposite directions is, on average, rather high. As a consequence, a generic vehicle riding in one of the two opposite directions crosses with a relatively high frequency vehicles riding in the opposite direction. This high frequency of crossings between vehicles riding in opposite directions makes communication collisions relatively highly probable: each crossing is in fact potentially dangerous (from the viewpoint of the communications network service quality), because the two vehicles, seen as nodes of the ad-hoc communication network, may exploit the same communication channel; if this happens, a service interruption is experienced by at least one of the two nodes.
In M. Gerla, “Clustering and Routing in Large Ad Hoc Wireless Nets”, Final Report 1998-99 for MICRO project 98-044, that can be found at www.ucop.edu/research/micro/98—99/98—044.pdf, and in M. Gerla et al, “On Demand Routing in Large Ad Hoc Wireless Networks with Passive Clustering”, Proceedings of IEEE Wireless Communication and Network Conference, September 2000, hierarchical clustering schemes for ad-hoc networks are dealt with.